Parcel Proxy https://github.com/parcel-bundler/parcel/issues/55

server.js

const express = require('express')
const proxy = require('http-proxy-middleware')
const Bundler = require('parcel-bundler')

const bundlerOptions = { cache: false }
const proxyOptions = { target: 'http://localhost:3000' }

const app = express()
const bundler = new Bundler('src/index.html', bundlerOptions)

app.use('/api', proxy(proxyOptions))
app.use(bundler.middleware())

app.listen(Number(process.env.PORT || 1234))